Anglican and Roman Catholic authorities in Britain issued on Tuesday official declarations stating their firm opposition to government plans to change the legal definition of marriage, saying that despite reassurances claiming otherwise, clergy may very well be forced to perform same-sex marriages.

"The uniqueness of the institution of marriage is based on the fact that the human person exists as both male and female and that their union for the purpose of procreation, mutual support and love has, over the centuries of human history, formed a stable unit which we call the family," said the Catholic Bishops' Conference of England and Wales in a statement.

The U.K. government has been deliberating whether to change the traditional definition of marriage to include same-sex couples by 2015, a proposal backed by British Prime Minister David Cameron. Same-sex civil partnerships have been legal in England since 2005.

"Conservatives believe in the ties that bind us; that society is stronger when we make vows to each other and support each other. So I don't support gay marriage despite being a Conservative. I support gay marriage because I'm a Conservative," Cameron said back in Oct. 2011 during a Conservative party conference.

"To change the nature of marriage for everyone will be divisive and deliver no obvious legal gain given the rights already conferred by civil partnerships," said the Church of England, Mother Church of the Anglican Communion. "We believe that imposing for essentially ideological reasons a new meaning on a term as familiar and fundamental as marriage would be deeply unwise."

The U.K government has said that it will allow exemptions for churches that do not wish to marry same-sex couples -- "We won't be asking anybody to do anything that goes against their conscience," Home Secretary Theresa May, whose department is leading the consultation, previously remarked.

But the Church of England reportedly has expressed fears that such plans are "unlikely to survive legal challenges in domestic and European courts," meaning that clergy might have to perform such ceremonies anyway, or risk breaking the law.

Tory, or conservative, Member of Parliament Crispin Blunt told the BBC that the government's plan to "to protect, indeed proscribe, religious organizations from performing same-sex marriages" may be "problematic legally."

Human rights lawyer Lucy Scott-Moncrieff, vice president of the Law Society, said in another interview with the BBC that it was unlikely the European Court of Justice will force religious institutions to perform marriages against their will, "but what it might say is that religious organizations should be allowed to if they want to," she argued.

Some pro-gay organizations have accused the church of focusing less on real important issues and diverting most of its attention to same-sex marriage.

"Many bishops in the Church of England today will be rather pleased because once again they are not talking about global poverty or the HIV pandemic, they are talking about the subject that obsesses them, and that is sex," responded Ben Summerskill, chief executive of the gay rights organization Stonewall.

The Catholic Church in England and Wales is further asking believers to sign a petition the Coalition for Marriage website, which has so far gathered over 500,000 signatures opposing the government's plans to change the definition of marriage.

"Throughout history and in virtually all human societies marriage has always been the union of a man and a woman. Marriage reflects the complementary natures of men and women. Although death and divorce may prevent it, the evidence shows that children do best with a married mother and a father," the Coalition for Marriage argues.